1. Field of the Invention
This invention relates to a cooling system for cooling an object to a very low temperature, such as a refrigerant-filled chamber type refrigerator, a superconducting magnet apparatus, etc.
2. Description of the Related Art
It is known that most of superconducting magnet apparatuses now put to practice employ both an immersion cooling system wherein a superconducting coil and a cryogenic refrigerant such as liquid helium are contained in an adiabatic container, and a system wherein a thermal shield embedded in an adiabatic layer of the adiabatic container is cooled by a cryogenic refrigerator. Moreover, a superconducting magnet apparatus which employs a refrigerator direct cooling system is now being developed. In this apparatus, a superconducting coil contained in an adiabatic container is directly cooled by a cryogenic refrigerator.
Many of such superconducting magnet apparatuses use a refrigerant-filled chamber type refrigerator as a cryogenic refrigerator, on the grounds that it has a small size and can realize a sufficiently low temperature. The refrigerant-filled chamber type refrigerator usually employs a plural-stage expansion type cooling system with a plurality of refrigerant-filled chambers, and a gas control system for introducing gas of high pressure into the overall cooling system and exhausting gas therefrom in an alternate manner. A typical refrigerant-filled chamber type refrigerator employs the Gifford-McMahon refrigerating cycle.
To cool a superconducting coil less than its critical temperature by a refrigerator direct cooling method, a refrigerant-filled chamber type refrigerator which employs a two-stage expansion system is usually used. This refrigerator cools the thermal shield to about 50 K. in a first cooling stage, and the superconducting coil to about 5 K. in a second cooling stage.
It is desirable to minimize the pre-cooling time required to cool the superconducting coil less than its critical temperature. In the case of using the refrigerant-filled chamber type refrigerator constructed as above, however, more than 50 hours are usually required for the following reason:
FIG. 1 shows examples of cooling capacities of the first and second cooling stages in the refrigerant-filled chamber type refrigerator, which employs the two-stage expansion system and the Gifford-McMahon refrigerating cycle. As is evident from FIG. 1, the cooling capacity of the first or second cooling stage is substantially constant within a range of from the room temperature (300 K.) to about 100 K. In other words, in this temperature range, the required pre-cooling time cannot be shortened even when a large amount of gas is supplied from the gas control system, since the amount of heat absorption is inherently limited.